Glossary
A
Absolute (or Kelvin) temperature scale
is used for
the temperature in all calculations involving T. The unit is the kelvin (K). The average kinetic energy of the molecules in a system is directly proportional to its temperature in kelvins.
Absolute zero
is 0 K, which is -273.16
oC. It is the
temperature at which molecules have no kinetic energy.
Absorbance
is a measure of the amount of light
absorbed by a substance. The absorbance of a solution depends upon both the concentration and the molar absorptivity of the absorbing substance at the wavelength of the light, and the distance through the solution that the light travels. See Beer’s Law.
Absorption
of a photon increases the energy of an
atom or a molecule by the energy of the photon (h
ν). A photon can be absorbed only if its energy
matches the energy differe
nce between two energy
levels in the atom or molecule.
An
absorption spectrum
presents the absorbance of a
substance as a function of the wavelength or frequency of light.
The
acceptor orbital
is the orbital on an oxidizing
agent that accepts the transferred electrons in a redox reaction.
The
acid dissociation or ionization constant
is the
equilibrium constant for the
reaction of an acid with
water: HA + H
O 2
→
H
O 3
1+ + A
1-.
An
acidic salt
is a salt in which the acidity of the
cation is greater than the basicity of the anion.
An
acidic solution
is one with [H
O 3
1+] > [OH
1-]. As a
result, pH < 7.0 at 25
oC for acidic solutions.
The
activation energy
is the energy of the transition
state relative to that of th
e reactants or products. It
is the minimum energy that the reactants must have in order for a reaction to occur.
An
active electrode
is one that is a participant in a
reaction. For example, a
copper electrode in a Cu
2+^
+ 2e
1-^
→
Cu half-cell is active because copper
metal participates in the reaction.
The
activity
is the ratio of the concentration of a
substance to its concentration in the standard state. It is unitless. The activities of pure solids and liquids are unity. The activity of a gas equals the partial pressure of the gas in atmospheres divided by 1 atm, while the activity of a solute equals its molar concentration divided by 1 M.
Addition polymers
are formed by addition reactions.
An
addition reaction
is a reaction in which two
reactants combine to fo
rm a single product.
Adhesive forces
are forces between different
molecules (compare with cohesive force).
An
alcohol
is a compound with the general formula R-
OH, where R is a generic group of atoms and OH is the hydroxyl group.
An
alkali metal
is an element that belongs to Group
1A.
An
alkaline earth meta
l is an element that belongs to
Group 2A.
An
alkane
is a saturated hydrocarbon,
i.e
., a
hydrocarbon that contains no multiple bonds.
An
alkene
is a hydrocarbon that contains carbon-
carbon double bonds.
An
alkyl group
is an organic group formed by
removing one hydrogen atom from an alkane
Allotropes
are different crystalline forms of the same
element that have different properties. Graphite and diamond are allotropes of carbon.
Alpha decay
is the emission of an alpha particle. It is
common among the heavy isotopes because it is the best way to reduce mass.
An
alpha particle
is a helium nucleus.
An
amide
is an amine attached to a carbonyl.
An
amine
is an ammonia molecule in which one or
more of the hydrogen atoms have been replaced with other groups.
An
amino acid
is a compound that contains both amine
and carboxylic acid functional groups.
Amorphous solids
have ordered arrangements of
particles over short distances only. This is referred to as local order.
The
ampere (A)
is the SI unit for electrical current. 1
A = 1C/s.
An
amphiprotic
substance is able to function as either
an acid or a base.
An
analyte
is a substance that is being analyzed
The
angstrom
(Å) is 10
-10
m. It is commonly used for
bond lengths because most bond lengths are between 1 and 2 Å.
Glossary
© by
North
Carolina
State
University
